What is nitrogen?
Nitrogen is an essential nutrient for plant growth and a natural part of freshwater ecosystems. In aquatic systems, nitrogen commonly occurs as nitrate (NO₃⁻), ammonium (NH₄⁺), or organic nitrogen.
However, excess nitrogen can degrade water quality, contribute to algal growth, and affect ecosystem health.
Where does nitrogen in freshwater come from?
Nitrogen enters freshwater from both natural processes and human activities. Under natural conditions, nitrogen cycling results in low concentrations in freshwater.
In Southland, rainfall and minimally impacted catchments contain low nitrogen concentrations. Deep glacial lakes such as Te Anau and Manapōuri have total nitrogen concentrations typically below 0.1 mg/L, reflecting natural baseline conditions.
Elevated nitrogen concentrations in rivers and groundwater are strongly associated with land-use intensity. Livestock urine deposition is the dominant source of nitrogen loading to soils in grazed systems, and nitrogen fertiliser use contributes to the overall nitrogen pool available for plant uptake or loss. When nitrogen inputs exceed plant uptake capacity, nitrate can leach beyond the root zone and enter groundwater, which may later discharge to rivers and streams.
Diffuse nitrogen losses occur across large areas of land rather than from a single discharge point, making them more complex to manage.
How does nitrogen affect people and our environment?
Nitrogen can also contaminate drinking water, causing serious illness, and posing severe risk to some people including infants and the elderly. It can also pose a risk to pets and stock.
Both common forms of nitrogen in water – nitrate and ammonia – cause problems. Nitrate can kill sensitive organisms like young fish species. Ammonia is highly toxic to fish and other creatures that live in water, so direct discharge of ammonia-rich wastes such as raw sewage or dairy shed effluent can be particularly damaging.
Excess nitrogen in waterways also causes damage to ecosystems by promoting the overgrowth of large plants called macrophytes, and tiny algae called periphyton and phytoplankton.
Excess nitrogen can:
- Increase algal growth in rivers and lakes.
- Reduce water clarity and aesthetic values.
- Alter aquatic ecosystems and invertebrate communities.
- Contribute to nitrate contamination of groundwater.
Nitrogen science
We test monthly for the two forms of nitrogen that get into water (nitrates & ammonia) at a number of groundwater bores, and surface water sites (rivers, streams, lakes) around Southland.
Environment Southland has completed scientific modelling, using the latest information and data, to better understand the gap between where our water quality is now and where it needs to be to meet our goals for improved freshwater quality - the Murihiku Southland draft freshwater objectives.
Several modelling studies have been undertaken to predict (estimate) the size of load reductions needed to achieve the objectives related to nitrogen in rivers, lakes and estuaries.
The estimated reduction in nitrogen load to achieve the national bottom lines is 47%. To achieve the proposed Southland Water and Land Plan standards the reduction required is 66%, and to achieve the draft Murihiku Southland freshwater objectives it is 70%.
Read the report here > Report - Nutrient load reduction modelling - lakes, rivers, estuaries (LWP) (PDF, 3.3MB)
Find out more about this work and our programme to achieve the improvements in water quality needed.
There are a number of things you can do to take action. Find out more about what you can do.
